Multiple-conductor electrical cable.



C. W. DAVIS.

MULTIPLE CONDUCTOR ELECTRICAL CABLE.

APPLICATION FILED JAN. 14, 1914.

Patented Mar. 16, 1915.

UNITED STATES PATENT OFFICE.

CHARLES W. DAVIS, OF EDGEWORTH, PENNSYLVANIA, ASSIGNOR TO STANDARD UNDERGROUND CABLE COMPANY, OF PITTSBURGH, PENNSYLVANIA, A CORPO- RATION OF PENNSYLVANIA.

MULTIPLE-CONDUCTOR ELECTRICAL CABLE.

Specification of Letters Patent.

Patented Mar. 16, 1915.

T a all whom it may concern:

Be it known that 1, CHARLES W. DAvis, residing at Edgeworth, in the county of Allegheny and State of Pennsylvania, a citizen of the United States, have invented or discovered certain new and useful Improvements in Multiple-Conductor Electrical Cables, of which improvements the following is a specification.

Multiple-conductor cables for the transmission of electric currents are ordinarlly built up by wrapping or otherwise applying to the. individual conductors (whether solid or stranded) at body of insulating material in the form of an annular coating of uniform depth; the individual conductors, thus severally insulated are then gathered and twisted into a rope; a filling material is used. filling the interstices of the rope and rounding 1t superficially to substantially cylindrical form and then the cable is sheathed or otherwise protected and prepared for its intended service. The filling material ordinarily used is jute fiber. This jute filler has heretofore been regarded merely in the light of a mechanical filling material, adopted because it is cheap and light and, so far as concerns any difiiculty, disadvantage, or danger of destruction incident to the passage of electric current in the conductors, the bodiesof insulating material surrounding the individual conductors, have been looked to, and they alone have been looked to, for prevention.

My invention proceeds fromthe discovery that the jute filler, is, under ordinary conditions, itself liable to destruction because of the heating eflccts of high-tension currents flowing in the conductors of the cable; and the perception that this jute filler, cheap as it is, may to the advantage of both cablebuilder and user be replaced by another filler of a' different material which, although perhaps more costly, will by virtue of its properties when under electric stress not be subject to destruction, as the jute filler is.

I have stated that. my invention is aplicable to multiple-conductor cables; I shall in the ensuing description direct particular attention tov three-conductor cables, and it will be understood that the precision of my description, in this particular is not precision by way of limitation, but only by way 05 defimteness in illustration.

The invention is applicable generally to multiple-conductor cables, whatever be the specific; number of conductors composing them.

When a three-conductor cable such as is now ordinarily used is operated on threephase currents at voltages of a few thousand volts and higher, the loss in the dielectric due to the electrical stress alone, or the dielectric loss added to the heat due to current flow, makes the center of the cable its hottest part. In a cable constructed in the usual manner, with a central filling of jute fiber, the liability of the filler to be destroyed by the heat generated in its substance is such as to forbid the use of the cable to carry currents of otherwise possible voltage, and this would be true even if the filler were more resistant to electric stress than it is. The danger of destruction Wullld still prevent the use of the cable at an efli ciency otherwise possible. My discovery is first that this central body is to be regarded, not primarily as a filler, to occupy a space formed in the mechanical make-up of the cable, but it is to be regarded primarily as an insulator; and, when so regarded, possibilities open of increasing the efficiency of the structure as a whole. I' use other material than jute fiber and material of much greater specific inductive capacity. Unless the specific inductive capacity be made very much greater than that of the-insulation built around the individual conductors. this central body will necessarily be subject to a strain as great or greater than that to which the neighboring bodies of conductor insulation are subjected. \Vhen. as in the now prevailing practice. the individual conductors are insulated with paper and the central filler is jute, the jute filler is subjected to a voltage stress which soon rises to a point beyond the endurance of the filler. Besides this, the electric field in the space between the conductors is in reality a rotary electrostatic field and the insulating material lying in this region is, apparently subject at times to superimposed stresses of a greater frequency than that of the supply voltage. Since dielectric loss in good solid dielectrics is roughly proportional to the square of the voltage and directly proportional to the frequency of the applied electric stress, the central jute filler and the edges of the jute laterals lying nearest this central region are subjected to an environment which may and often does overheat, and thus carbonize and destroy them.

The first feature of my invention, then, is to remove the jute filler ordinarily employed in the center of the cable and to replace it with a body of material whose specific inductive capacity is high, and preferably as great or greater than that of the neighboring bodies of insulation surrounding the individual conductors. Specifically, I preferably employ a body of rolled or wrapped paper, impregnated with an insulating compound of great inductive capacity, that is to say, greater than the induction capacity of the material with which the paper wrappings of the individual conductors are impregnated.

lteferring to the drawings, Figure 1 illustrates in cross-section a three-conductor cable embodying my invention. The individual conductors, 1, are shown to be stranded, but that is immaterial here. Each conductor is enveloped ina body of insulation 2, and at the center, and filling the space between, is the closely wrapped, impregnated and shaped body 3, the nature of which I have indicated, and the presence of which constitutes the first feature of my present invention. The efl'ect of the presence of this central and highly resistant body is to cause the insulation surrounding the individual conductors to do more in the way of resisting stress. The stress at the center is constantly varying, and the body 3 should therefore be equally strong to resist electric stress in all-directions. This desideratum is approximated in the spirally wound paper. It would be more nearly approximated in a homogeneous body, such as rubber or one of the high-melting-point and more or less elastic and tenacious hydrocarbon compounds, such as ozite. Such material as this could be shaped before appli- ,cation, by forcing through a die. While,

therefore, I have shown and described tightly rolled paper as the material preferred, I prefer it only in view of the relative ease of production and application, and I include in the scope of my invention the use of materials of great specific. inductive capacity, generally, having instanced others which, from theoretical considerations alone; are even better than the material to which I give preference. The rolled-paper filler may be prepared'by rolling the paper, impregnating it with hot and fluid compound, pressing it to the final shape, in which it is allowed to cool and harden. I have spoken of the specific material used for filling this central space as preferably a material of high specific inductive capacity and have mentioned paper, rubber, and hydro-carbon compounds. It will be understood that if the filler in the form of roller paper, for instance, should be coated with an integument of conducting materialtin foil for instance-the specific inductive capacity of the filler would be increased to infinity; and, so far as concerns the stress that I am speaking of, and the danger of a destructive generation of heat, the presence of such a foil would be absolutel controlling to prevent such generation. he presence of such a film of metal as I here suggest may afiord advantages over and above those herein set forthadvantages which I have described and claimed in other applications of mine, to wit, Ialpplication filed November 4th, 1913, Serial 0. 799,090, and another application filed on even date herewith, Serial No. 811,980. Consideration of these advantages need not be introduced here. I further find it desirable to eliminate lateral weak spots present in a jute-filled cable, replacing the bodies of jute which ordinarily lie in the external grooves between the grouped con ductors with properly shaped bodies of material of high specific inductive capacity. Wrapped and impregnated paper may be used, as already described for the central space, and such material is shown at 5, Fig. 1. It will be understood that, as with the central body, other highly resistant material mav be employed.

of destruction of inferior material by heat, nor because the stress is constantly shifting, but because the material employed should be able to resist stress equally in all directions. And, inasmuch, as the strains to which, in high-tension work, these lateral This superior material 7 13 used, not so much because of the danger fillers are subjected are unbalancedwhereas the strains are at the center of the cable balanced-I consider it desirable (though not essential to my invention in its broader aspect) that the-inductive capacity of these laterals be greater even. than that of the central filler.

The further features of my invention proceed from the discovery of the fact that in the insulation of the individual conductors there is, in the prevalent practice, poor economy. When the conductors are assembled and the cable is in service, the dielec tric strains set up are reatest toward the center of the cable and. east toward the periphery. The thicki'icss of insulation there fore around the individual conductor requisite toward the center is unnecessarily great toward the periphery. In other words, if the conductor be centered in its body of insulation (as is now universally the case) the depth of insulation required on one side is far greater than conditions of service require on the other side. possible and desirable, and herein 'lies a feature of my invention, to separate the individual conductors more widely, within 9; ca-

It is, ,therefore, I

ble of given over-all diameter, by arranging each individualconductor eccentrically within its body of insulation, and grouping the insulated conductors with the greater depth of insulation inward and the less depth outward. This is illustrated in Fig. 1 of the drawings, where it will be perceived that the conductors 1 lie eccentrically within their ovate envelops of insulation, and as widely separated one from the other as this ovate shape permits. This shaping may readily be accomplished in the manner suggested in Fig. 1 and more fully illustrated in Fig. 2, which is a perspective view of thejndividual conductor and shows it diagrammatically in course of being enveloped in insulation. The form of insulation here shown is wrapped-on paper, and theovate form is produced by inter-leaving strips of paper 6 between successive wrappings. This specific way of building the insulation is illustrative, but does not limit my broader claims.

I claim as my invention:

1. In a multi le-conductor cable, the combination of a p urality of conductors and a lurality of individually separate filler Bodies of which one forms a central and the other lateral fillers, bunched within a protecting cover, the said central filler capable of enduring dielectric stress equally irrall directions.

2. In a multiple-conductor cable, the combination of a plurality of conductors inclosed each in insulation and a plurality of individually separate filler bodies of which one forms a central and the other lateral fillers, all bunched within a protecting cover, the said central filler having a speclfic impedance not greater than that of the insulation around the individual conductors.

3. In a multiple-conductor cable, the combination of a plurality of individually insulated conductors grouped about a central body whose specific inductive capacity is reater than that of the insulation of the individual conductors.

4. In a multiple-conductor cable, the combination of a plurality of conductors grouped about a central body formed of wrapped paper.

5. In a multiple-conductor cable the combination of a plurality of three or more conductors each surrounded individually in an envelop of insulation and all grouped about a central body of insulating material, bodies of insulating material filling the lateral interstices between adjacent insulated conductors, the material composing such lateral fillers being of a specific inductive capacity greater than that of the material employed in insulating the several conductors individually.

6. In a multiple-conductor cable, the combination of a plurality of individually insulated conductors grouped about a common center, and bodies of material filling the lateral interstices between adjacent insulated conductors, the filling material so employed being of specific inductive capacity greater than that of the insulation which surrounds the conductors individually.

7. In a multiple conductor cable the combination of a plurality of three or more conductors each surrounded individually in an envelop of insulation and all grouped about a central body of insulating material, bodies of insulating material filling the lateral interstices between adjacent insulated conductors, the material composing such lateral fillers being of a specific inductive capacity greater. than that of the material employed in insulatin the several conductors individually, and teria(li oiwhicktlwsaid' central filler is compose 8. In a multiple-conductor cable, the com- I bination of a plurality of conductors within an inclosing casing, the entire space around said conductors and within said casing being occupied with impregnated bodies of closely wrapped paper.

9. In a multiple-conductor cable a plu rality of conductors each arranged eccentrically within an envelop of insulation and all being grouped with the greatest depth of insulating envelop disposed toward the center of the group.

10. A conductor inclosed in a body of wrapped-on paper insulation, strips of paper longitudinally arranged on one side of the conductor being included between successive wrappings.

In testimony whereof I have hereunto set my hand.

CHARLES w. DAVIS. Witnesses E. S. SUNKINS, FRANCIS J TOMABSON.

greater than that of themes- 

